Why are sky textures not showing up?
This appears to be due to a several things:


Transparent is enabled in Render settings > Film.
This renders the image with alpha transparency instead of sky. Useful for compositing, but hides the sky texture (at least in viewport render).
Ray visibilities are disabled on the world material.
The compositing setup doesn't take into account the sky texture. This is understandable, as a real-life camera would over-expose the sky when taking a picture with a reasonably exposed interior.


So to make this work, you'll need to modify the compositing setup.



Start by adding your sky texture to the world material. Ensure the strength is set correctly and at least Camera ray visibility is enabled:



Disabling Properties > Render > Film > Transparent won't work in this particular case, since the compositing setup uses it create glare from the sky. Instead you can use the Environment pass:


Enable the environment pass in Properties > Renderlayers > Passes:


Connect the environment pass output of the _mainScene Render layers node to the Alpha over node in the COLOR CORRECTION frame:


You may have to reduce the amount of glare by reducing the Fac value of this mix node:




Render: